a history of science-4-第23节

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nt；　by　division　of　its　substance；　of　other　generations　of　nuclei。　The　word　cell　received　a　final　definition　as　〃a　small　mass　of　protoplasm　supplied　with　a　nucleus。〃

In　this　widened　and　culminating　general　view　of　the　cell　theory　it　became　clear　that　every　animate　organism；　animal　or　vegetable；　is　but　a　cluster　of　nucleated　cells；　all　of　which；　in　each　individual　case；　are　the　direct　descendants　of　a　single　primordial　cell　of　the　ovum。　In　the　developed　individuals　of　higher　organisms　the　successive　generations　of　cells　become　marvellously　diversified　in　form　and　in　specific　functions；　there　is　a　wonderful　division　of　labor；　special　functions　being　chiefly　relegated　to　definite　groups　of　cells；　but　from　first　to　last　there　is　no　function　developed　that　is　not　present；　in　a　primitive　way；　in　every　cell；　however　isolated；　nor　does　the　developed　cell；　however　specialized；　ever　forget　altogether　any　one　of　its　primordial　functions　or　capacities。　All　physiology；　then；　properly　interpreted；　becomes　merely　a　study　of　cellular　activities；　and　the　development　of　the　cell　theory　takes　its　place　as　the　great　central　generalization　in　physiology　of　the　nineteenth　century。　　Something　of　the　later　developments　of　this　theory　we　shall　see　in　another　connection。


ANIMAL　CHEMISTRY

Just　at　the　time　when　the　microscope　was　opening　up　the　paths　that　were　to　lead　to　the　wonderful　cell　theory；　another　novel　line　of　interrogation　of　the　living　organism　was　being　put　forward　by　a　different　set　of　observers。　Two　great　schools　of　physiological　chemistry　had　arisenone　under　guidance　of　Liebig　and　Wohler；　in　Germany；　the　other　dominated　by　the　great　French　master　Jean　Baptiste　Dumas。　　Liebig　had　at　one　time　contemplated　the　study　of　medicine；　and　Dumas　had　achieved　distinction　in　connection　with　Prevost；　at　Geneva；　in　the　field　of　pure　physiology　before　he　turned　his　attention　especially　to　chemistry。　　Both　these　masters；　therefore；　and　Wohler　as　well；　found　absorbing　interest　in　those　phases　of　chemistry　that　have　to　do　with　the　functions　of　living　tissues；　and　it　was　largely　through　their　efforts　and　the　labors　of　their　followers　that　the　prevalent　idea　that　vital　processes　are　dominated　by　unique　laws　was　discarded　and　physiology　was　brought　within　the　recognized　province　of　the　chemist。　So　at　about　the　time　when　the　microscope　had　taught　that　the　cell　is　the　really　essential　structure　of　the　living　organism；　the　chemists　had　come　to　understand　that　every　function　of　the　organism　is　really　the　expression　of　a　chemical　changethat　each　cell　is；　in　short；　a　miniature　chemical　laboratory。　And　it　was　this　combined　point　of　view　of　anatomist　and　chemist；　this　union　of　hitherto　dissociated　forces；　that　made　possible　the　inroads　into　the　unexplored　fields　of　physiology　that　were　effected　towards　the　middle　of　the　nineteenth　century。

One　of　the　first　subjects　reinvestigated　and　brought　to　proximal　solution　was　the　long…mooted　question　of　the　digestion　of　foods。　Spallanzani　and　Hunter　had　shown　in　the　previous　century　that　digestion　is　in　some　sort　a　solution　of　foods；　but　little　advance　was　made　upon　their　work　until　1824；　when　Prout　detected　the　presence　of　hydrochloric　acid　in　the　gastric　juice。　A　decade　later　Sprott　and　Boyd　detected　the　existence　of　peculiar　glands　in　the　gastric　mucous　membrane；　and　Cagniard　la　Tour　and　Schwann　independently　discovered　that　the　really　active　principle　of　the　gastric　juice　is　a　substance　which　was　named　pepsin；　and　which　was　shown　by　Schwann　to　be　active　in　the　presence　of　hydrochloric　acid。

Almost　coincidently；　in　1836；　it　was　discovered　by　Purkinje　and　Pappenheim　that　another　organ　than　the　stomachnamely；　the　pancreashas　a　share　in　digestion；　and　in　the　course　of　the　ensuing　decade　it　came　to　be　known；　through　the　efforts　of　Eberle；　Valentin；　and　Claude　Bernard；　that　this　organ　is　all…important　in　the　digestion　of　starchy　and　fatty　foods。　It　was　found；　too；　that　the　liver　and　the　intestinal　glands　have　each　an　important　share　in　the　work　of　preparing　foods　for　absorption；　as　also　has　the　salivathat；　in　short；　a　coalition　of　forces　is　necessary　for　the　digestion　of　all　ordinary　foods　taken　into　the　stomach。

And　the　chemists　soon　discovered　that　in　each　one　of　the　essential　digestive　juices　there　is　at　least　one　substance　having　certain　resemblances　to　pepsin；　though　acting　on　different　kinds　of　food。　　The　point　of　resemblance　between　all　these　essential　digestive　agents　is　that　each　has　the　remarkable　property　of　acting　on　relatively　enormous　quantities　of　the　substance　which　it　can　digest　without　itself　being　destroyed　or　apparently　even　altered。　In　virtue　of　this　strange　property；　pepsin　and　the　allied　substances　were　spoken　of　as　ferments；　but　more　recently　it　is　customary　to　distinguish　them　from　such　organized　ferments　as　yeast　by　designating　them　enzymes。　The　isolation　of　these　enzymes；　and　an　appreciation　of　their　mode　of　action；　mark　a　long　step　towards　the　solution　of　the　riddle　of　digestion；　but　it　must　be　added　that　we　are　still　quite　in　the　dark　as　to　the　real　ultimate　nature　of　their　strange　activity。

In　a　comprehensive　view；　the　digestive　organs；　taken　as　a　whole；　are　a　gateway　between　the　outside　world　and　the　more　intimate　cells　of　the　organism。　　Another　equally　important　gateway　is　furnished　by　the　lungs；　and　here　also　there　was　much　obscurity　about　the　exact　method　of　functioning　at　the　time　of　the　revival　of　physiological　chemistry。　That　oxygen　is　consumed　and　carbonic　acid　given　off　during　respiration　the　chemists　of　the　age　of　Priestley　and　Lavoisier　had　indeed　made　clear；　but　the　mistaken　notion　prevailed　that　it　was　in　the　lungs　themselves　that　the　important　burning　of　fuel　occurs；　of　which　carbonic　acid　is　a　chief　product。　　But　now　that　attention　had　been　called　to　the　importance　of　the　ultimate　cell；　this　misconception　could　not　long　hold　its　ground；　and　as　early　as　1842　Liebig；　in　the　course　of　his　studies　of　animal　heat；　became　convinced　that　it　is　not　in　the　lungs；　but　in　the　ultimate　tissues　to　which　they　are　tributary；　that　the　true　consumption　of　fuel　takes　place。　Reviving　Lavoisier's　idea；　with　modifications　and　additions；　Liebig　contended；　and　in　the　face　of　opposition　finally　demonstrated；　that　the　source　of　animal　heat　is　really　the　consumption　of　the　fuel　taken　in　through　the　stomach　and　the　lungs。　　He　showed　that　all　the　activities　of　life　are　really　the　product　of　energy　liberated　solely　through　destructive　processes；　amounting；　broadly　speaking；　to　combustion　occurring　in　the　ultimate　cells　of　the　organism。　Here　is　his　argument：


LIEBIG　ON　ANIMAL　HEAT

〃The　oxygen　taken　into　the　system　is　taken　out　again　in　the　same　forms；　whether　in　summer　or　in　winter；　hence　we　expire　more　carbon　in　cold　weather；　and　when　the　barometer　is　high；　than　we　do　in　warm　weather；　and　we　must　consume　more　or　less　carbon　in　our　food　in　the　same　proportion；　in　Sweden　more　than　in　Sicily；　and　in　our　more　temperate　climate　a　full　eighth　more　in　winter　than　in　summer。

〃Even　when　we　consume　equal　weights　of　food　in　cold　and　warm　countries；　infinite　wisdom　has　so　arranged　that　the　articles　of　food　in　different　climates　are　most　unequal　in　the　proportion　of　carbon　they　contain。　The　fruits　on　which　the　natives　of　the　South　prefer　to　feed　do　not　in　the　fresh　state　contain　more　than　twelve　per　cent。　of　carbon；　while　the　blubber　and　train…oil　used　by　the　inhabitants　of　the　arctic　regions　contain　from　sixty…six　to　eighty　per　cent。　of　carbon。

〃It　is　no　difficult　matter；　in　warm　climates；　to　study　moderation　in　eating；　and　men　can　bear　hunger　for　a　long　time　under　the　equator；　but　cold　and　hunger　united　very　soon　exhaust　the　body。

〃The　mutual　action　between　the　elements　of　the　food　and　the　oxygen　conveyed　by　the　circulation　of　the　blood　to　every　part　of　the　body　is　the　source　of　animal　heat。

〃All　living　creatures　whose　existence　depends　on　the　absorption　of　oxygen　possess　within　themselves　a　source　of　heat　independent　of　surrounding　objects。

〃This　truth　applies　to　all　animals；　and　extends　besides　to　the　germination　of　seeds；　to　the　flowering　of　plants；　and　to　the　maturation　of　fruits。　It　is　only　in　those　parts　of　the　body　to　which　arterial　blood；　and　with　it　the　oxygen　absorbed　in　respiration；　is　conveyed　that　heat　is　produced。　Hair；　wool；　or　feathers　do　not　possess　an　elevated　temperature。　This　high　temperature　of　the　animal　body；　or；　as　it　may　be　called；　disengagement　of　heat；　is　uniformly　and　under　all　circumstances　the　result　of　the　combination　of　combustible　substance　with　oxygen。

〃In　whatever　way　carbon　may　combine　with　oxygen；　the　act　of　combination　cannot　take　place　without　the　disengagement　of　heat。　It　is　a　matter　of　indifference　whether　the　combination　takes　place　rapidly　or　slowly；　at　a　high　or　at　a　low　temperature；　the　amount　of　heat　liberated　is　a　constant　quantity。　The　carbon　of　the　food；　which　is　converted　into　carbonic　acid　within　the　body；　must　give　out　exactly　as　much　heat　as　if　it　had　been　directly　burned　in　the　air　or　in　oxygen　gas；　the　only　difference　is　that　the　amount　of　heat　produced　is　diffused　over　unequal　times。　In　oxygen　the　combustion　is　more　rapid　and　the　heat　more　intense；　in　air　it　is　slower；　the　temperature　is　not　so　high；　but　it　continues　longer。

〃It　is